Very good question!
I wrote a bit about advantages at the bottom of this:
The intent behind DPI-RAI is not to limit it to DPI but rather be a credit facility for multiple indexes (and maybe even DeFi tokens).
The Index community controls the parameters set for each collateral and can also add new collateral types faster than any external credit facility.
If the plan was to allow only DPI holders to lever then I agree it would be easier to just add it in a separate credit facility. This proposal outlines a way bigger vision though where you have your own credit pipeline that INDEX can use to solidify its position in the market.